Whirlpool bath. Is there legal requirement for additional dedicated RCD even if we have Double RCD CU?

[benhurr]

No it isn't, it is evidence that different people have different opinions on going beyond the requirements of the regulations.

Ask any one of those electricians to show you the requirement for a second 30mA RCD connected in series in the regulations and they will not be able to.

Would you put a second rcd for whirlpool bath on already protected by cu-rcd circuit?

 

[RichardM]

There is no requirement for a second RCD in fact it makes it more inconvenient for the occupier as both RCDs will probably trip at the same time. If you are concerned about tripping half the house, assuming it's a separate circuit for the bath disconnect the circuit at the C/U on the RCD rail and fit an RCBO. Socket type RCDs even 10mA will not necessarily protect the main RCD, believe me discrimination in RCDs / RCBOs is more luck than science.

 

[davesparks]

Would you put a second rcd for whirlpool bath on already protected by cu-rcd circuit?

No I wouldn't because the second one would be redundant. And depending on the exact make and model used could cause more hassle than good.

If one was installed then the upstream RCD (at the CU) will trip first because the normal cumulative leakage current of the installation will bias it to trip first.

If the one installed downstream is the usual SFCU type then, experience has shown, that pressing the test button on it often causes the RCD at the CU to trip first. This test button is often the only means of switching it off so any time you want to turn it off half the house goes off.

 

[brianmoooore]

If one was installed then the upstream RCD (at the CU) will trip first because the normal cumulative leakage current of the installation will bias it to trip first.

The tripping tolerance of RCDs is +0%/-50%, meaning a 30mA RCD is within tolerance as long as it trips in the required time when subjected to anywhere between 15mA and 30mA of leakage.
There will be a balance of probability that the upstream RCD will trip if considering a large number of installations, but for any individual installation, this cannot be assumed. The tolerance of sensitivity is likely to be greater than any cumulative leakage.